Image rejector circuit for radio receivers



R. B. ALBREGHT IMAGE REJECTOR CIRCUIT FOR RADIO RECEIVERS Filed Jan. 5,1938 AM a; 2/0 510$ W A a M Q Patented May 21, 1940 ATE T OFHE IMAGEREJECTOR CIRCUIT FOR RADIO RECEIVERS Robert Benjamin Albri signer toght, Philadelphia, Pa, as- .Philco Radio and Television Corporation,Philadelphia, Pa, a corporation of Dela- Ware Application January 5,

1938, Serial No. 183,542

In Great Britain January 9, 1937 10 Claims.

This invention relates to novel means for suppressing or attenuatingundesired signals in radio receiving systems and, more particularly, tomeans for attenuating so-called image signals in radio receivers of thesuperheterodyne type.

The principal object of the invention is to provide a novel device ofthis character which may be employed to advantage in a superheterodynemultiband receiver, that is a receiver adapted to receive signals in aplurality of frequency bands, and which device embodies a minimum numberof circuit elements and utilizes certain elements in a dual manner.

Another object of this invention is to provide a multiband receiverofthe superheterodynetype in which the coil or coils of an unused waveband are employed to attenuate undesired image signals associated withthe desired signals in the particular wave band being used, and in whichthe same coils are used for the transfer of desired signals in theirparticular bands, thus permitting the coils to serve a dual purpose.

Still another object of the invention is to provide a novel antennacircuit for a radio receiver of this type, which circuit has associatedwith it the novel image suppression device provided by.

the invention.

Although the principles of the invention are applicable in any instancewhere it is desired to attenuate undesired signals and to transferdesired signals substantially unaffected, as stated above, the inventionis particularly adapted for use in a superheterodyne multiband radioreceiver and, therefore, it will be described herein with particularreference to such specific application.

As is now well known, any superheterodyne receiver is responsive tosignals of two different frequencies which represent the sum anddifference, respectively, of the oscillator frequency and theintermediate frequency of the receiver. To a lesser extent, the receiveris also responsive to signals whose frequencies represent the sum anddifference, respectively, of the intermediate free quency and harmonicsof the oscillator frequency. One of the signals to which the receiver isresponsive is commonly known as the image signal. The presence of theimage signal is particularly undesirable in certain instances, forexample in the reception of signals in the long wave band, that issignals having a frequency in the frequency range between and 350kilocycles, since the image signals corresponding thereto lie in thestandard broadcast band which extends from 550 kilocycles to 1600kilocycles within which there are present signals emanating frompowerful transmitting stations. For example, if the intermediatefrequency of a receiver is 450 kilocycles, the image frequency rangecorresponding to signals in the long wave band will extend from 750kilocycles to 1250 kilocycles. Therefore, in order to receive signals inthe long Wave band efficiently, it is necessary that the strong imagesignals present in the broadcast band be' effectively attenuated.

The present invention accomplishes this desired result by providing asignal transfer path between the antenna and first tuned circuit whichpath includes an impedance in shunt across the path which impedance hasa high value and therefore good transfer characteristics for desiredsignals but a low value and therefore poor transfer characteristics forthe undesired signals. Moreover, the invention utilizes for this purposecertain elements which are used for reception of signals in thebroadcast band, thus causing such elements to serve a dual purpose. Inthis manner, the desired image suppression is effected employing aminimum number of parts or elements.

Other objects and features of the invention will be apparent fromconsideration of the drawing and specification. The single figure of thedrawing is a diagrammatic illustration of one embodiment of theinvention.

Referring now to the drawing, there is shown a multi-band radio receiverwhich is adapted to receive signals in the usual frequency bands, viz.,the long wave band, the broadcast band and the short wave band. To thisend, there are provided three tuning coils L1, L2 and L3 which arerespectively designed to be tuned to signals in the long wave band, thebroadcast band and the short wave band by means of the tuning condenserC: and the padding condensers Cpl, C 2 and 0133 associated respectivelywith the coils L1, L2 and La. By means of the three-position switch S1,any one of these coils may be selected to adapt the receiver forreception of signals in a particular frequency band, as will be wellunderstood, As illustrated, the selectable contacts I, 2 and 3 of theswitch S1 are connected respectively, to the coils L1, L2 and L3.Preferably there should be some coupling between L1 and L2 as indicatedby M in the figure. Suitable grid bias or automatic volume control biasmay be supplied to the grid of the control tube V by means of theisolating resistance R connected, to the common point of L1, L2 and L3as shown.

Signals may be supplied to the several tuned circuits either from aconventional high impedance antenna A or from a low impedance sourcesuch as the transmission line T. L. which may be connected to anassociated antenna as well understood. When it is desired to receivesignals from the antenna A, the antenna may be connected to the terminal4 and the signal transfer will then take place through the capacitycoupling comprising condensers C1, C2 and Ca. When it is desired toreceive signals from the transmission line T. L., the line may beconnected to the terminals 5 and 6, which are connected to the primarywinding P of transformer T, the midpoint of the primary winding beingconnected to ground so as to balance the transmission line. Secondary Sof the transformer has one end coninected to ground and its other endconnected to a point between condensers C2 and C3 so that thetransformer secondary is shunted about the condenser C3. The signaltransfer takes place through the coupling transformer T which builds upa signal across the condenser C; from whence the signal is supplied tothe selected tuned circuit.

The wave band switch S1 is connected to the input element of a vacuumtube V so that the switch is adapted to connect the selected tunedcircuit to the said input element. The tube V may be a conventionaldetector-oscillator or it may be a conventional tube of the mixer type.In any case, there will be associated with the tube V and oscillator 0having a tuning condenser Cu, which condenser may be ganged with thetuning condenser Ct for simultaneous operation of the two tuningcondensers, as indicated by the broken-line representation. It will beunderstood that the oscillator is a conventional device which requiresno further illustration than the diagrammatic representation of thedrawing.

The output of the tube V may be supplied to any suitable utilizationmeans such as the intermediate frequency amplifier of a conventionalsuperheterodyne radio receiver.

In accordance with the present invention, there is provided a secondswitch S2 which may be a three-position switch similar to the switch S1as illustrated, although only one position of switch S2 is utilized inthe manner to be presently described. Preferably, the switch S1 and S2are ganged or connected together for simultaneous operation as indicatedby the broken-line representation. The stationary contact I of switch S2is connected to the stationary contact 2 of switch S1 as illustrated. Acondenser C4 has one side connected to the movable arm of the switch S2and its other side connected to ground. It will be seen, therefore, thatwhen the switch S2 is in its position I, the coil L2 is connected to thecondenser C4. When the switch S2 is in either of its positions 2 and 3,however, it is ineffective since its contacts 2 and 3 are dead contacts.

Suppose now that it is desired to receive signals in the long wave band,as described above, the switch S1 will be thrown to its position I so asto select the long wave band coil L1. At the same time, the switch S2will, of course, be thrown to its position I, thus connecting thecondenser C4 to the broadcast band coil L2. The coil L1 is now energizedby the signal across the condensers C2 and C3. It will be noted,however, that the series circuit comprising coil L2 and condenser C4 isplaced in shunt relation with the condensers C2 and C3. Remembering thatthe coils L1 and L2 are adapted respectively to be tuned to receivesignals in the long wave band and the broadcast band by means of thecommon tuning condenser Ct, it will now be apparent that the condenserC4 may be designed in such relation to the coil L2 that the seriescircuit L204 may be made to attenuate image signals in the broadcastband. By designing the condenser C4 so that it has a capacity equal toabout one-half the maximum capacity of the tuning condenser Cr, theseries circuit L2C4 may be made resonant at a frequency in the lower endof the image frequency band associated with the long wave hand. Forexample, the resonant frequency may be in the neighborhood of 750 k. 0.At low frequencies corresponding to desired signals in the low wave band(i. e., -350 k. c.), the series circuit will have a very high capacitivereactance and hence will not deleteriously effect the transfer ofdesired signals. With respect toimage signals, however, the seriescircuit will substantially completely suppress those signals having afrequency in the lower part of the image frequency range since, forsignals of such frequencies (1. e., near the resonant frequency ofL204), the impedance of the series circuit L204 is very small. Forundesired signals in the high end of the image frequency band, theimpedance of condensers C2 and C3 will be small and will prevent thetransfer of such signals to a considerable extent. However, by couplingL1 and L2 slightly in accordance with the invention, further suppressionof undesired image signals in the high part of the image signal band maybe had. This obtains from the fact that for these higher imagefrequencies, the image frequency current in L2 will induce a voltage inL1 which will oppose the undesired image signal in that circuit.Complete balancing out may be obtained at one frequency, for example,the middle of the image band, and for the remainder of the band the twosignals will oppose one another and nearly completely balance out.

For reception of signals in the broadcast band, the switches S1 and S2are moved to position 2 and the circuit L2Ct may be tuned to the desiredsignal in the broadcast band in the normal manner, Reception of signalsin the short wave band likewise obtains by moving S1 and S2 to position3. It will be understood of course that the padding condensers Cpl, CD2and C 3 will be adjusted so that the tuning condenser Cr will track ineach band.

Generally speaking, it has been found that by using the circuit of theinvention as above described, it is necessary to provide only two tuningcondensers instead of three such condensers as used heretofore to obtainsatisfactory image signal attenuation. Thus, in this respect also theinvention simplifies the radio receiver and effects economy in themanufacture thereof.

In a specific embodiment of the invention, the following values werefound to be satisfactory. The coil L1 may have an inductance of 2580microhenries. The coil L2 may have an inductance of 221 microhenries.The coil L3 may have an inductance of 1.9 microhenries. The tuningcondenser Cr may be of sufficient capacity to tune these coils over theabove-mentioned frequency bands. The condensers C1, C2 and C3 may eachhave a capacity of .01 microfarad. The tube V may be of any suitabletype such as the 6A'7G frequency converter. or a simple tetrode. Theintermediate frequency of the radio receiver may be 451 kilocycles.

The coils L1 and L2 may be wound on the same form. In the above specificembodiment, L1 comprises 365 turns of 3-40 Litzendraht wire and L2comprises 82 turns of 7-41 Litzendraht wire, both windings being of theuniversal type and in the same direction. The coils were wound on a formand spaced The beginnings of each winding were connected together and toC2. The end of the winding L1 was connected to Ct and the end of thewinding L2 to C4, by means of the- I switches S1 and S2, respectively.

While the invention has been described with respect to a singleembodiment thereof, it will be understood that various modifications maybe made as will be apparent to those skilled in the art and consequentlythe invention is not limited to the specific form shown in the figure.

I claim:

1. In a multiband superheterodyne radio receiver, a signal channel, avtuning condenser, a coil tunable to a desired signal in one wave band bysaid condenser, a second coil tunable to a desired signal in a differentwave band by said condenser, each of said coils having one end connectedin the signal channel, a wave band switch for connecting the other endof one coil in the signal channel for transferring desired signals inits Wave band and for alternatively connecting the other end of thesecond coil in the signal channel for the transfer of desired signals inthe second wave band, a condenser of predetermined capacity relative tothe inductance of one of said coils, and means including a switch forserially connecting said last-mentioned coil and con,- denser in shuntwith the signal channel to suppress undesired signals associated withthe desired signals. upon operation of said wave band switch to connectthe first-mentioned coil in the signal channel.

2. In a multiband superheterodyne radio receiver, a signal channel, atuning condenser, a coil tunable to a desired signal in one wave band bysaid condenser, a second coil tunable to a desired signal in a differentwave band by said condenser, each of said coils having one end connectedin the signal channel, a. wave band switch for connecting the other endof one coil in the signal channel for transferring desired signals inits wave band and for alternatively connecting the other end of thesecond coil in the signal channel for the transfer of desired signals inthe second wave band, a condenser of predetermined capacity relative tothe inductance of one of said coils, and means including a switch forserially connecting said last-mentioned coil and condenser in shunt withthe signal channel to suppress undesired signals associated with thedesired signals upon operation of said wave band switch to connect thefirst-mentioned coil in the signal channel, said last-mentionedcondenser having a value between the minimum and maximum values of saidtuning condenser.

3. In a multiband superheterodyne radio receiver, a source of signals, atuning condenser, a plurality of coils, each of said coils being tunableby said tuning condenser to transfer desired signals in different wavehands, a common connection between one end of each of said coils,connections for transferring signals from said source to said commonconnection, an amplifier, means for connecting the tuning condenser inshunt with the input circuit of said amplifier, a wave band switch foralternatively connecting the other ends of said coils to the tuningcondenser to transfer desired signals in the respective wave bands, acondenser of predetermined capacity relative to the inductance of one ofsaid coils, and switch means for serially connecting last-mentioned coiland condenser in a shunt path across said signal source upon operationof said wave band switch to connect another of said coils in the signalchannel, the said seriesconnected elements in said shunt path beingresonant to undesired signals associated with the desired signalstransferred by said other coil.

4. In a multiband superheterodyne radio receiver, a source of signals, atuning condenser, a plurality of coils, each of said coils being tunableby said tuning condenser to transfer desired signals in different wavebands, there being some mutual coupling between said coils, a commonconnection between one end of each of said coils, connections fortransferring signals from said source to said common connection, anamplifier, means for connecting the tuning condenser in shunt with theinput circuit of said amplifier, a wave band switch for alternativelyconnecting the other ends of said coils to the tuning condenser totransfer desired signals in the respective wave bands, a condenser ofpredetermined capacity relative to the inductance of one of said coils,and switch means for serially connecting said last-mentioned coil andcondenser in a shunt path across said signal source upon operation ofsaid wave .band switch to connect another of said coils in the signalchannel, the said seriesconnected elements in said shunt path beingresonant to undersired signals associated-with the desired signalstransferred by said other coil.

5. In a multiband radio receiver, a source of signals, a vacuum tubehaving at least a cathode, a control grid, and an anode, a tuning con-.denser connected between said cathode and said control grid, a pluralityof coils, each of said coils being tunable by said tuning condenser tosignals in different wave bands, a capacitor of predetermined capacityrelative to the inductance of one of said coils, and switching means forconnecting another of said coils serially between said signal source andsaid control grid, while simultaneously connecting said one coil andsaid capacitor in series across said signal source.

6. In a multiband radio receiver, a source of signals, said sourcehaving a high potential side and a low potential side, a vacuum tubehaving at least a cathode, anode and control grid, a tuning condenserconnected between said cathode and said control grid, a connectionbetween the low potential side of said signal source and said cathode, afirst coil capable of being tuned over a certain frequency band by saidcondenser, a second coil capable of being tuned over a second frequencyband by said condenser, switching means for selectively connectingeither one of said coils serially between the high potential side ofsaid signal source and said control grid, whereby the selected coil maybe tuned to series resonance by said tuning condenser, a capacitorhaving a predetermined capacity relative to the inductance of saidsecond coil, and additional switching means operative when said firstcoil is switched into circuit for connecting said second coil and saidcapacitor in series across said signal source.

7. In a multiband superheterodyne radio receiver, a source of signals,said source having a high potential side and a low potential side, avacuum tube having at least a cathode, anode and control grid, a tuningcondenser connected between said cathode and said control grid, aconnection between the low potential side of said signal source and saidcathode, a first coil capable of being tuned over a certain frequencyband by said condenser, a second coil capable of being tuned over asecond frequency band by said condenser, switching means for selectivelyconnecting either one of said coils serially between the high potentialside of said signal source and said control grid, whereby the selectedcoil may be tuned to series resonance by said tuning condenser, acapacitor having a predetermined capacity relative to the inductance ofsaid second coil, and additional switching means operative when saidfirst coil is switched into circuit for connecting said second coil andsaid capacitor in series across said signal source, said second coil andsaid capacitor eing adjusted to give series resonance at some point inthe image frequency band associated with the band covered by said firstcoil.

8. In a multiband radio receiver, a source of signals, said sourcehaving a high potential side and a low potential side, a vacuum tubehaving at least a cathode, anode and. control grid, 9, tuning condenserconnected between said cathode and said control grid, a connectionbetween the low potential side of said signal source and said cathode, afirst coil capable of being tuned over a certain frequency band by saidcondenser, a second coil capable of being tuned over a second frequencyband by said condenser, said coils being inductively coupled, switchingmeans for selectively connecting either one of said coils seriallybetween the high potential side of said signal source and said controlgrid, whereby the selected coil may be tuned to series resonance by saidtuning condenser, a capacitor having a predetermined capacity relativeto the inductance of said second coil, and additional switching meansoperative when said first coil is switched into circuit for connectingsaid second coil and said capacitor in series across said signal source.

9. In a multiband radio receiver, a source of signals, said sourcehaving a high potential side and a low potential side, signal transfermeans having a high potential side and a low potential side, a tuningcondenser connected between the high and low potential sides of saidtransfer means, a connection between the low potential side of saidsource and the low potential side of said transfer means, a plurality ofcoils, means connecting one end of each coil to the high potential sideof said source, switching means for selectively connecting the otherends of said coils to the high potential side of said transfer means,the said coils being tunable respectively by said condenser overdifferent frequency hands, a capacitor having a predetermined capacityrelative to the inductance of one of said coils, and means for seriallyconnecting said capacitor and said one coil in shunt with said sourceupon operation of said switching means to connect another of said coilsto said transfer means.

10. In a multiband radio receiver, a source of signals, signal transfermeans, a tuning condenser connected across said transfer means, aconnection between one side of said source and one side of said transfermeans, a plurality of coils, means connecting one end of each coil tothe other side of said source, switching means for selectivelyconnecting the other ends of said coils to the other side of saidtransfer means, the said coils being tunable respectively by saidcondenser over different frequency bands, a capacitor having apredetermined capacity relative to the inductance of one of said coils,and means [or serially connecting said capacitor and said one coil inshunt with said source upon operation of said switching means to connectanother of said coils to said transfer means.

ROBERT BENJAMIN ALBRIGHT.

